The present invention related generally to electronic engineering and, more particularly, to a surface-acoustic-wave (SAW) bandpass filter.
Known in the art is a SAW bandpass filter having a piezoelectric substrate with input and output electroacoustic transducers in series on its surface. At least one of the transducers is apodized, and a shield, which is actually a strip of metal film, is located between the transducers (U.S. Pat. No. 4,438,417).
In this prior art filter the apodization boundaries of the apodized transducer are symmetric about the longitudinal axis of the transducer, which is parallel to the propagation direction of the surface acoustic waves. Sum bus bars, which are strips of metal film respectively connected with a plurality of electrode fingers, are also aligned in parallel to the longitudinal axis of the transducer.
This embodiment of an apodized transducer has large inoperative areas of electrode surfaces (from apodization boundaries to sum bus bars), which produce a high level of SAW reflection, resulting, in its turn, in distortion of the transducer impulse response and, eventually, in shape distortion of the filter amplitude--frequency response over its transmission band frequency range.
Also known in the art is a SAW bandpass filter having a piezoelectric substrate with, in series on its surface, an input apodized electroacoustic transducer with sum bus bars connected to filter input terminals, a shield and an output non-apodized acoustoelectrical transducer with sum bus bars connected to filter output terminals (V. I. Rechitsky "Acoustoelectronic Radio Components", Radio i Sviaz, Moscow, 1987, p.p. 36, 37, FIG. 1.22).
Apodization boundaries of the apodized transducer in this prior art filter, as in the first above-mentioned filter, are located symmetrically about the longitudinal axis of the transducer, which is parallel to the propagation direction of the surface acoustic waves. However, the sum bus bars in this transducer are inclined and the space between the apodization boundary and the sum bus bars is filled with a conductive coating material. As a result, the layout of the side of the conductive coating material on the side of the apodized transducer facing the non-apodized transducer can provide a condition ensuring minimum distortions along the SAW front.
In designing this layout, different speed values of the SAW propagation should be taken into account along three specific zones or areas, such as a zone of overlapping electrode fingers connected to the opposite sum bus bars, a zone of the conductive coating (e.g., metallization) material and a free-surface zone of the piezoelectric substrate. To ensure the equality of the relative phase incursions of the surface acoustic waves during the SAW propagation from the overlapping-electrode-finger zone (apodization boundary) to the transducer boundary, the following conditions should be met: the phase incursion of the wave propagating in the overlapping-electrode-finger zone should be equal to the sum of the wave phase incursions in the conductive-coating zone and the free-surface zone.
In this embodiment of an apodized transducer, due to the reduction of the inoperative zones, the level of SAW reflection from them and, accordingly, the distortions of the transducer impulse response resulting from these reflections have been decreased.
However, the embodiment still has a high level of SAW reflection from the overlapping-electrode-finger zone due to a long length of signal passage (equal to the length of a transducer) in the overlapping-electrode-finger zone. Because the apodization axis is aligned with the axis of symmetry of the apodized transducer, the level of signal distortion from SAW reflection from the overlapping-electrode-finger zone changes along the wave propagation front, i.e. diminishes in the directions from the longitudinal axis of the transducer towards its periphery. This degrades the quality characteristics of the SAW filter.
Also known in the art is an interdigital electroacoustic transducer with a sloping structure of apodization, i.e., in which the boundaries of the apodization are symmetric about a line passing across the transducer diagonally. In this embodiment, the path of signal passage in the overlapping-electrode-finger zone has been considerably reduced, which decreases the level of SAW reflection (A. J. Vigil et. al. "A Study of the Effects of Apodized Structure Geometries on SAW Filter Parameters", Proceedings of the 1987 Lithium IEEE Ultrasonic Symposium, p. 139, 140, FIG. 2).
However, the use of a transducer with a sloping structure of apodization in a SAW filter is hindered by the following factors. First, the availability of large areas of inoperative electrodes results in a high level of SAW reflection accompanied by initiation of side components of the SAW filter frequency response and, thus, degraded quality characteristics. Second, polarization dependence results, i.e. the level of the direct-passage signal depends on the polarity of the apodized transducer connection. The latter is caused by a considerable difference in the length of the finger electrodes connected with sum bus bars of the different polarities.